During the past two decades, Pseudomonas aeruginosa has been recognized as a pathogen which causes between 10% and 20% of infections in most hospitals. Pseudomonas infection is especially prevalent among patients with burn wounds, cystic fibrosis, acute leukemia, organ transplants, and intravenous-drug addiction. P. aeruginosa is a common nosocomial contaminant, and epidemics have been traced to many items in the hospital environment. Patients who are hospitalized for extended periods are frequently affected by this organism and are at increased risk of developing infection. The most serious infections include malignant-external otitis, endophthalmitis, endoconditis, meningitis, pneumonia, and septicemia. The likelihood of recovery from Pseudomonas infection is related to the severity of the patient's underlying disease process. The reported mortality for P. aeruginosa pneumonia is as high as 50-80%. Even with the development of newer antibiotics, resistance remains a problem necessitating combined antibiotic treatment for severe P. aeruginosa infections.
Alternative therapy for the management of severe P. aeruginosa infections have been evaluated for many years. Immunotherapy has been the alternative most extensively explored. In this area, attention has focussed on the virulence factors. As with most bacterial pathogens, virulence of Pseudomonas aeruginosa is multifactorial and is the product of many interacting variables, involving both the bacterium and the host.
Evidence suggests that the initial event in infection is the adherence of microorganisms to epithelial cells of mucosal surfaces [E.H. Blackey, J. Infect. Dis., 143:325-345 (1981)]. Organisms that are unable to adhere to mucosal surfaces fail to colonize because they are removed by the secretions that bathe the mucosal surfaces. The adherence process is dependent upon the specific recognition between bacteria and epithelial cells. For a number of gram-negative bacteria, including P. aeruginosa, attention has been directed to surface appendages as mediations of adherence. These surface appendages are termed `adhesins`, and the distribution of specific receptors for adhesins determines many of the tissue tropisms noted for bacteria. In the case of P. aeruginosa, polar pili present on the surface of the organism have been shown to mediate adherence to buccal epithelial cells. The evidence for this is as follows: (1) nonpiliated strains do not adhere to epithelial cells; (2) protease treatment of P. aeruginosa drastically reduces the ability of these organisms to adhere to epithelial cells; (3) preincubation of epithelial cells with purified pili significantly decreases the adherence of intact organisms, and (4) antibody to purified pili prevents the adherence of organisms to buccal epithelial cells.
Although P. aeruginosa pili are antigenically heterogeneous in different clinical isolates, there is evidence that a portion of the pilus is conserved [see Paranchych et al., Antibiotics Chemother. 36:49-57 (1985)]. As this common domain is important in binding to epithelial cells [see Doig et al., Infection and Immun., 56: 1641-1646 (1988)], it is useful in the production of a broadly effective P. aeruginosa pili vaccine.
The surface of many gram-negative bacteria, e.g., E. coli, P. aeruginosa M. bovis, N. gonorrhea, are covered with filamentous structures called pili or fimbriae. Pili are composed primarily of protein (pilin) and have been found to act as antigenic determinants when injected into test animals. Certain pili, including PAO, PAK, CD4, as they are commonly referred to, and others, mediate the colonization of P. aeruginosa in humans. Some bacterial cells lacking these pili, either through mutation or loss of the plasmid carrying the pilus gene, are incapable of colonizing mucosa. Apparently, the pili on the surface of the bacterium adhere to the lining of the throat and trachea through specific interactions with epithelial cell receptors. P. aeruginosa can utilize both pili and alginate (the principle component of the P. aeruginosa capsule) as adhesins to mediate attachment to human respiratory epithelial cells [see Doig et al., Infection and Immun., 55: 1517-1522 (1987); Doig et al., Infection and Immun., 56: 1641-1646 (1988); Marcus et al., Infection and Immun., 47:723-729 (1985); Ramphal et al., Infection and Immun. 44: 38-40 (1984); Ramphal et al., Infection and Immun., 47:1-4 (1985); Woods et al., Infection and Immun., 29: 1146-1151 (1980).
Equilibrium analysis of P. aeruginosa binding to human respiratory epithelial cells indicates that the Pseudomonas pilus adhesin has a considerably higher apparent affinity or binding constant (Ka) than does the alginate adhesin [McEachran et al., Can. J. Microbiol. 31:563-569 (1985), McEachran et al., J. Microbiol. Meth., 5: 99-111 (1986); Doig et al., Infection and Immun., 55: 1517-1522 (1987)].
These observations suggest that the pilus adhesin is likely the dominant Pseudomonas adhesin in the initiation of an infection. Adhesin-mediated anchorage is a prerequisite for the induction of disease by P. aeruginosa.
Anything which would biologically interfere with this adhesion should be effective in blocking infection. Such a technique has been investigated by monoclonal antibody treatment of bacterial adhesion and has been reported in the patent literature, e.g., U.S. Pat. No. 4,443,549 and U.S. Pat. No. 4,702,911 and published U.S. PCT application Ser. No. PCT/US85/00565. It is important to note that bacterial adhesins are unique so that this technique is not predictable with various other bacteria.